wallpapers/image: fix potential invalid slide index in `nextSlide`

If the last wallpaper is deleted before `nextSlide` is called, the next
time `nextSlide` is called, m_currentSlide is always increased by 1,
which is an invalid index, causing the slideshow always stuck at the
last wallpaper.

CCBUG: 473088


(cherry picked from commit e69e0a93)